Focusing a laser to a very small dot

[kecked]

Build an interferometer. Just couple mirrors and no lens. You can measure nm or less of movement. It’s not as easy as I say to get going but you can do it. look Up Michelson Morley interferometer.

 

[bostjan]

You should be able to get a single mode, low power laser module with a focusable lens for pretty cheap. You could end up spending much more on study mounts than you would on the laser itself under these circumstances, though.

If weight is a huge concern, those study mounts are going to be the biggest difficulty there as well.

But you didn't ask about mounts; the only question you asked the forum:

Do you know which stores sell this kind of device and I don't need to select the optics myself?

Amazon sells everything you should need for cheap. You could get nicer stuff from Thorlabs, but it'd likely blow way past your budget. I honestly have no idea what you are trying to implement, though. Good luck.

 

[kecked]

If your device is in motion consider LiDAR.

 

[Commander J. Bloodmaker]

I think @kecked is on the right track here. Light weight, accurate, and feasible for just about anyone willing to learn how to build one. No special lenses or anything needed. While more complex than slap together, it is easily within the reach of a college student, cheap to make.